“…The method of [11] is applicable to an operator of a form more general than it is the case in [1,3,4], but there also the explicit Fourier representation of the main part is needed. The analysis of the methods of [1,8,11,12] is given with respect to Sobolev norms, and the maximum norm error estimates of optimal order, which means the convergence of the same order as for trigonometric interpolation, are not achieved. In [3, 4] Hölder-Zygmund norms are used, and pointwise error estimates of optimal order are derived for the methods involved in the case of operators of integer order.…”